A heating device, based on the use of heat propagating by radiation (reference is then made to radiant heat) starting from a radiant element (notably manufactured from a metal alloy or ceramic), is in particular used in heat treatment furnaces within which products, typically metal products (such as bars, tubes, strips) are treated, but also, for example, products made from other materials, such as ceramics.
“Indirect heating” means that the heating is not done directly between the heat source (flame in the case of combustion) and the product to be treated.
Document FR 1,315,564 discloses one particular type of heating device having an elliptical shape, i.e., a shape made up of an infinity of arcs of circle and that can be obtained, by definition, as an envelope of the family of circles whereof the diameters are the parallel support chords of a given circle. An ellipse is in fact a closed plane curve generated by a point so moving that its distance from a fixed point divided by its distance from a fixed line is a positive constant less than 1.
Generally, the heat treatment furnaces comprise an entire series of radiant elements placed above one another and/or next to one another in vertical and/or horizontal rows. Generally, the products to be treated travel vertically and/or horizontally opposite these elements and/or between these elements, from which the radiant heat is emitted. To that end, each radiant element comprises at least one heat source, which may for example assume the form of a burner provided with at least one combustible product injector, at least one combustive inlet and at least one combustion gas outlet such that, supplied with a combustible product and a combustive, the burner develops a flame within the radiant element from which the heat then radiates toward the products to be treated.
Primarily, the radiant elements currently used and known from the state of the art consist of radiant tubes, different shapes of which are proposed. For example, a first type of radiant tube is the “W” tube, which is made up of four strands with a circular section, and a second type of radiant tube is the “double P” type, which is made up of three strands with substantially circular hollow sections. However, other forms of radiant elements have been proposed, for example radiant cartridges (see below).
In the heat treatment processes done by continuous or non-continuous travel of products (for example, metal strips) in front of radiant elements positioned in a furnace, the heat transfer depends on the overall heat emitting surface area (i.e., all of the surface areas of the radiant elements from which heat is radiated), the view factor (or shape factor) and the difference of the temperature (as characterized by the Stefan Boltzmann law on radiation transfers) between the radiant surface and the product to be treated.
It should be noted that, by definition, the view factor or shape factor makes it possible to define the proportion of the total flow (of heat) emitted by a first surface (S1) and arriving on a second surface (S2). In practice, the overall heat-emitting surface is formed by a series of parallel tubes generally placed transversely relative to the movement direction of the product, in the case of travel.
These tubes, installed according to practice in a furnace in vertical and/or horizontal rows, radiate heat toward the product, but at the same time, due to their side-by-side position and/or positioning above one another, the same radiant tubes radiate one another (mutual radiations). Indeed, significant surface areas of successive radiant tubes face one another, a surface of a given radiant element intercepting the radiation of another given successive radiant tube, this not making it possible to ensure optimal heating of the product, but leading to mutual overheating of the tubes, which, in certain scenarios, transmit heat to one another by radiation.
On one hand, this results in limiting the transfer of heat toward the product to be treated, since a non-negligible quantity of heat is prevented from being sent toward the latter. Indeed, when two radiant elements/tubes known from the state of the art follow one another (are placed side by side or above one another), they “bother” one another, and a loss of effective radiation surface area is observed for each of these radiant elements/tubes. Typically, this mutual radiation phenomenon causes a loss of heating capacity of the successive radiant elements/tubes, i.e., tubes placed side by side or above one another.
On the other hand, the shapes currently known for the radiant tubes of the state of the art contribute to the creation of a temperature gradient along the radiant tubes, this temperature gradient in particular having a considerable impact on the longevity of the radiant tubes.
In summary, in practice, with such radiant elements in the form of tubes, several non-negligible problems are therefore encountered, in particular including the presence of a temperature gradient along the tubes, but also the phenomenon of mutual radiation between successive tubes, which is responsible for loss of heating capacity of the product to be treated by each of the tubes. This loss of heating capacity is related to the presence of many radiant tubes in the furnaces and the small amount of free space between the latter. Indeed, there should be enough tubes to reach a sufficient heating capacity, but a large number of tubes amplifies the mutual radiation phenomenon. This results in a deterioration of the shape factor (or view factor) towards the product to be treated (strip, etc.), which is then not optimal for heating by radiation. Indeed, with the radiant elements known from the state of the art, the fraction of the radiation emitted by an element/tube and intercepted by the surface of another element/tube is non-negligible, which results in a low view factor value toward the product to be treated and a high view factor value between radiant elements (tubes or cartridges).
Although a view factor as large as possible is desirable between the radiant elements (tubes or housings) and an element to be treated, a view factor as small as possible is, on the contrary, desirable between the radiant elements (tubes or housings) following one another.
To try to address these drawbacks, document EP 1,203,921 proposes a device for indirect heating by radiation in the form of a radiant cartridge having a parallelepiped shape. More specifically, this cartridge contains a combustion channel, one end of which is connected to a burner supplied with a combustible product and a combustive by means of at least two injectors, the other end of the channel being open to allow a circulation of the combustion gases. A discharge of these combustion gases is provided via a combustion gas outlet present on the surface of the radiant cartridge. With such a heating device according to document EP 1,203,921, a flame is developed in a combustion tunnel that radiates heat toward the walls of the parallelepiped shape formed by the cartridge, those walls then in turn radiating heat toward the products to be treated.
Furthermore, although this prior document describes a heating device presented as having increased performance levels ensuring homogeneity of the temperature of the radiating walls of the cartridge and an improved view factor, the fact nevertheless remains that significant mutual radiation between the upper and lower surfaces of two successive parallelepiped cartridges is problematic, as is the case with traditional radiant tubes.